The present invention relates to collector flights mounted on sludge collector mechanisms in sedimentation tanks used in municipal and industrial waste treatment plants and in potable water treatment plants to collect and convey settled sludge as well as floating wastes to the point of ultimate disposal within the sedimentation tanks. More specifically, the invention discloses a fiberglass-reinforced polymeric sludge collector flight which is not susceptible to either the corrosive forces found in waste water or the potentially damaging physical stresses encountered by longer flights used in sedimentation tanks with widths of approximately 20 feet.
Sludge collector mechanisms are used in the sedimentation tanks of wastewater and potable water treatment plants to scrape the settled sludge from the bottom of the tank and to skim the floating waste material from the surface of the waste water. Typically, sludge collectors include transverse flights which are carried on a pair of drive chains in a circuit along the bottom of the tank as well as along the surface of the water in the tank to accomplish the scraping and skimming functions.
The end regions of collector flights are equipped with means to mount the flights on the drive chains. The drive chains are looped around collector chain sprockets which are mounted on cross shafts, with the sprockets located near the side walls of the tank to minimize shaft deflection. However, a significant drawback of this arrangement is that collector flights have a substantial unsupported length between the chains which is subject to flexing and sagging forces from the sludge scraping and surface skimming process. Thus, it is preferable to have flights which are stiff enough to resist these forces yet are not prone to cracking or breaking.
Conventional flights have been made of wood, as disclosed by Walker in U.S. Pat. No. 2,393,725, or steel, as disclosed by Zack in U.S. Pat. No. 2,056,062 and Canadian Pat. No. 625,103 to Mans. Although these prior designs performed these intended functions fairly adequately, their drawbacks have been amplified as many plant operators replace heavy, corrosion-prone metal chain with lighter, non-corroding polymeric chain. Thus, in addition to the prior problems of wooden flights, which were subject to rotting and warping, and steel flights which corroded, when used in conjunction with polymeric chain these heavier flights put a strain on chain links which significantly decreases the usable life of those chains.
There have been attempts to fabricate polymeric collector flights, as disclosed in U.S. Pat. Nos. 3,313,422 to Swenson and 3,394,816 to Lowry, but these flights were purposely designed to be buoyant as a means of reducing the operating load of the metal chain equipped apparatus. Buoyant flights are incompatible with polymeric chain because the chain is not heavy enough to keep the flights in contact with the floor of the tank, as the prior metal chain did, and prevents the efficient collection of settled sludge.
Prior polymeric collector flights were also subject to sagging and flexing in the long unsupported area between the two chains.
In the case of 20 foot long flights, the chains mount approximately 2.5 feet from the ends of the flight, leaving approximately 15 feet of flight unsupported. To provide additional strength and buoyancy, the prior polymeric flights were comprised of hollow, rectangular channels having provisions for the insertion of rigid internal support members extending the length of the flight. Despite the addition of supplemental support members, the length of this unsupported span was too great for even the weight of the cast iron chain, and the buoyant center tended to lift upward, causing the flight to camber.
This center camber effect caused the undesirable result of severe abrasion of the ends of the flight which were dragged along the concrete floor of the tank. Eventually, the ends became so abraded that holes were created, after which entire chunks of the thin polymeric shell of the flight began to break off. Not only did this situation allow the build up of sludge in the center of the tank by virtue of the cambered flights not making adequate contact with the tank floor, but the creation of holes in the flight allowed sewage to seep into the flight and become trapped, where it putrified, causing a severe odor problem.
Attempts to solve the cambering problem usually involved the addition of heavier support members into the polymeric flights, which defeated the purpose of using a lightweight flight to buoy the heavy metal chain.
Thus, there is a definite need for a sludge collector flight which is non-buoyant, light enough to be compatible with polymeric chain, yet strong enough to withstand the mid-span stresses due to heavy sludge loads, and which allows for the escape of any sludge which becomes trapped in the interior of the flight.
It is therefore an objective of the present invention to provide a polymeric collector flight which is compatible with polymeric chain.
It is a further objective of the present invention to provide a polymeric collector flight which is non-buoyant.
It is a still further objective of the present invention to provide a polymeric collector flight which is capable of withstanding the force of excessive mid-span sludge load.
It is another objective of the present invention to provide a polymeric collector flight which allows the easy escape of internally trapped sludge.